HR: 17:45h
AN: V42G-07    [PDF]
TI: The structural evolution of carbonaceous material during metamorphism : a geothermometer
AU: * Beyssac, O
EM: Olivier.Beyssac@ens.fr
AF: Laboratoire de Geologie - Ecole Normale Superieure, 24 rue Lhomond, Paris, 75005 France
AU: Goffe, B
EM: goffe@geologie.ens.fr
AF: Laboratoire de Geologie - Ecole Normale Superieure, 24 rue Lhomond, Paris, 75005 France
AU: Brunet, F
EM: brunet@geologie.ens.fr
AF: Laboratoire de Geologie - Ecole Normale Superieure, 24 rue Lhomond, Paris, 75005 France
AU: Bollinger, L
EM: bollinge@gps.caltech.edu
AF: GPS Department - Caltech, 1200 E. California Bvd, Pasadena, CA 91125 United States
AU: Avouac, J
EM: avouac@gps.caltech.edu
AF: GPS Department - Caltech, 1200 E. California Bvd, Pasadena, CA 91125 United States
AU: Rouzaud, J
EM: rouzaud@cnrs-orleans.fr
AF: Centre de Recherche sur la Matiere Divisee - CNRS, 1b rue de la Ferollerie, Orleans, 45000 France
AB: With increasing metamorphic temperature, the organic matter present in sedimentary rocks is progressively transformed into graphite (graphitization). The degree of organization of this carbonaceous material (CM) as characterized by Raman spectroscopy (RSCM), can be used as a geothermometer which yields the maximum temperature reached during the metamorphic cycle (Beyssac et al., 2002). We used this RSCM geothermometer to map the maximum metamorphic temperatures through the Lesser Himalaya (LH) in Nepal. This study provides a large dataset (80 samples) to estimate uncertainty of this method and to ascertain its reliability by comparison with conventional petrological investigations. We show that the RSCM geothermometer might be used to detect inter-samples temperature variations as small as 10$\deg$C or so, but absolute temperatures are only loosely determined to +/- 50$\deg$C due to the uncertainty on the calibration. This successful application of the RSCM geothermometer confirms that, at the timescale of regional metamorphism (several My), the transformation of CM is mainly controlled by temperature. However, laboratory investigations suggest that, in addition to temperature, pressure should also play a role (Beyssac et al. 2003). As a matter of fact, high degree of organizations encountered in natural CM cannot be reproduced in laboratory without pressure, even at temperatures as high as 3000$\deg$C. In addition to the data acquired on natural CM, we will discuss laboratory experiments performed up to 8 GPa which show that (1) a few kbar of hydrostatic pressure are required to initiate microtextural and subsequent structural transformations within CM and (2) the overall effect of increasing pressure is to speed up graphitization process. Beyssac, O., Goffe, B., Chopin, C., and Rouzaud, J.N., 2002, Raman spectra of carbonaceous material in metasediments: a new geothermometer. Journal of Metamorphic Geology, 20, 859-871. Beyssac, O., Brunet, F., Petitet, J.P., Goffe, B., and Rouzaud, J.N., 2003, Experimental study of the microtextural and structural transformations of carbonaceous materials under pressure and temperature. European Journal of Mineralogy, in press.
DE: 1055 Organic geochemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting